It is known that bundles of electrical conductors, also known as cabling harnesses, can be subjected to a protective hardening operation corresponding to providing shielding against electromagnetic interference (EMI), in particular when they are used in civilian or military applications relating to aviation and space, or to shipping, and when mounted on board vehicles such as aircraft, ships, tanks, etc. . . . . Such a protective sheath serves to avoid malfunction of an electrical installation having various devices interconnected by such cable harnesses.
Naturally, the invention is not limited to this particular application and it relates more generally to making sheaths for improving the mechanical strength of elongate objects such as cables, etc.
In the specific application of the invention, in order to make such shielding, it is possible to perform braiding directly on harnesses, and thus on the various branches making up each harness, thereby producing sheaths that are obtained by braiding textile and/or metal strands and/or wires, as described for example in French patent applications Nos. 94 14968 and 94 14969 in the name of the Applicant.
Those shielding sheaths are made using a braiding machine of the kind described, for example, in French patent No. FR 2 742 772, also in the name of the Applicant, and comprising:                a bench through which said bundle for shielding can pass;        means for advancing said bundle along a braiding axis perpendicular to said bench;        a plurality of spindles mounted on supports regularly distributed on said bench around said passage, and carrying respective reels from which the braiding wires are entrained towards said bundle for braiding; and        drive means associated with said bench and suitable for driving said spindle supports along slideways provided in said bench.        
Thus, by causing the support and spindle assemblies to move circularly along the slideways around said passage while simultaneously actuating the advance means, the wires pulled from the reels by the bundle progressively build up the braid around said bundle. Such circular displacement of the spindle-and-support assemblies then enables substantially circular braids to be made. Nevertheless, appropriate routes for the slideways also make it possible to make braids around cross-sections that are I-shaped, T-shaped, . . . .
Although those machines give good results and are in widespread use, they nevertheless present certain drawbacks concerning more particularly their preparation and setting operations which are a function of the configuration to be given to the braiding, and also of the material that is to be used, and that also depend on the type of harness that is to be fitted with a braid.
Whenever the type and the shape of the braiding to be performed on a harness need to be modified, the resulting operations of preparing and setting the braiding machine turn out to be lengthy and tedious. For example, when a machine initially fitted with a certain number of spindles, each spindle carrying a reel of wire comprising a plurality of same-diameter strands, needs to be subjected to changes in the number of spindles and the associated reels in order to adapt to requirements, and in particular to the diameter of the harness that is to be shielded, to the braiding angles, and to the kind of wires, i.e. the nature of the material that is to be used for braiding, to the number of strands per wire, and to the diameters of said strands, the length of time taken to perform these operations is very penalizing for production. Thus, the time required for such an intervention can be as long as one hour or more.
Naturally, this occurs in theory for one particular application only, with the nature, the diameter, and the number of strands used for braiding being identical regardless of the number of spindles (or reels). Naturally, these characteristics differ as a function of different applications.
Thus, the potential number of interventions can be very large because of the number of setting parameters that need to be taken into account for the braiding machine, and in particular, the parameters listed below:                harness diameter;        nature of the material to be braided;        strand for braiding made of metal, textile, filled composite, metallized composite, . . . ;        strand diameter (or thickness, or width);        number of strands being braided: 3 to 11 strands per wire, for example, forming in this particular case nine groups of wires;        the number of spindles, i.e. of reels: 16, 32, 48, and 64 spindles, for example, forming in this particular case four groups of spindles; and        the braiding angle (lying in the range 10° to 80°, for example).        
It can thus be seen that these operations lead to a long down time for the machine, particularly when a large number of assemblies or of spindle and reel groups need to be changed with possible adjustments of braiding angle, with any down time reducing the overall productivity of the machine itself.
Furthermore, the setting parameters that are selected must be selected in highly rigorous manner in order to comply with electromagnetic protection requirements, particularly when it is understood that modern aircrafts, for example, are being fitted more and more with electronic means that provide an increasing number of functions in ever more automatic manner. Furthermore, these electronic means operate in environmental conditions that are more and more severe in terms of electromagnetic radiation, in particular because of the increasing use of structures made of composite materials that are more permeable to such radiations.
Consequently, the requirements for electromagnetic protection lead to optical coverage and transfer impedance thresholds that must be complied with by the shielding on bundles of electrical conductors.
In parallel, the weight of the shielding must be minimized. On this topic, it should be observed that the weight of the shielding depends in particular on its optical coverage percentage which is defined as being the ratio of the surface area of shielding on a harness (bundle of conductors) over the outside surface area of said harness.
Furthermore, transfer impedance represents linear electrical resistance as a function of the frequency of the electromagnetic radiation. The transfer impedance thus directly characterizes the effectiveness of shielding.
In this context, it should be recalled that two categories of electromagnetic radiation can be distinguished. The first category relates to electromagnetic radiation at a frequency lying in the range zero to 400 megahertz (MHz): such radiation is said to be of the conducted type since it is transmitted all the way to the ends of a harness and runs a risk of damaging items of equipment associated with said harness.
The second category relates to electromagnetic radiation for which energy is dissipated essentially by radiation along the length of the shielding: such radiation is at frequencies greater than 400 MHz. Under such circumstances, items of equipment associated with the harness are under threat only insofar as the higher the frequency, the nearer the electromagnetic attack must occur to said item of equipment. This means that it is possible to provide for a coverage percentage that is locally greater in the vicinity of items of equipment, while possibly reducing said coverage percentage over an ordinary portion of the harness, possibly to below a value that is conventional for such a location.
In an attempt to provide a solution to the problem of electromagnetic radiation of the conducted type, document U.S. Pat. No. 5,504,274 discloses a method of forming shielding that provides protection against electromagnetic radiation that is limited to frequencies of less than 50 MHz. Above that frequency, each item of equipment needs to possess its own protection, which is penalizing in terms of weight.